The use of thermoplastic lining of laminate plastics for protecting the laminates from degradation by chemicals and water is well known in the art. In particular, laminates such as glass reinforced polyester thermoset plastic have had acceptance in the manufacturing of pipes and piping systems for carrying water. Glass-reinforced polyester (GRP) thermoset pipe has been used widely in the making of pipes and storage vessels for liquids, including water, since the early 1970's. GRP pipes exhibit the characteristics of low thermal conductivity, resistance to corrosion and ease of installation; however, prolonged contact with water, especially heated water, can reduce the properties of GRP piping over a long period of time.
GRP laminate has been shown to become permeated by water after prolonged exposure to water through the absorption of the water into the plastics matrix, followed by diffusion of the water through the matrix of the resin. The water in contact with the resin acts as a lubricating agent and softens the resin matrix. Swelling of the plastic materials also occurs which is a function of the composition of the laminate. The small bubbles and occlusions present in the resin matrix become filled with the water causing local stresses that eventually produce micro-cracks in the laminate.
In order to minimize the corrosion of the laminate, impermeable liners have been used to slow the attack of water or other chemicals on the laminate. Thermoplastic liners are known in the art to have the following characteristics: (1) low water permeability; (2) high temperature resistance; (3) flexibility with age; (4) chemically inert to water; and (5) a good cost performance ratio. Even so, it has been reported that at elevated temperatures these thermoplastics are still somewhat permeable to water. If the permeability of the laminate pipe shell is less than the liner, then water will eventually accumulate by diffusion at the interface attacking the liner-laminate bond which will cause pocketing and blistering. If, on the other hand, the laminate is more permeable than the liner, the water will eventually diffuse completely through the pipe and the pipe will remain intact. Therefore, a liner needs to be constructed that is highly impermeable to water, yet is thick enough so that the diffusion rate of water is equivalent or lower to that of the laminate. C. Renaud, Influence of liner permeability on mechanical properties of GRP laminates after hot water exposure, I.R.P.I. pp.10-13, (January/February 1983). Furthermore, if the thermoplastic layer is of significant thickness, it will develop shear stresses between the liner and the shell which will exceed the bond strength and could cause the liner to pocket or blister.
There are examples of GRP laminates with thermoplastic liners in the prior art. For example, German patent DE3342386 A1 (1985) describes a tank constructed of multi-layer polyester/modified PPO for drinking water storage. At present, construction of GRP piping with thermoplastic liners involves a multi-step process where a thermoplastic resin is bonded to a pre-formed plastic shell using an adhesion promoting layer or some variation of this. Some examples of this technique are: U.S. Pat. No. 5,629,062 to Ejiri et al, "Fiber Reinforced Plastic Pipe and Process for Producing the Same", disclosing a fiber reinforced plastic pipe having an inner layer formed by winding a prepreg sheet containing a thermoset resin around a mandrel. U.S. Pat. No. 5,643,526, to Hert, et al., "Process for Preparing a Composite Tubular Article Consisting of a Vulcanized Elastomer Used in Combination with a Thermoplastic Containing Polyamide Blocks Especially Petro Pipe", describes a process for preparing a composite tubular article consisting of an inner layer of thermoplastic elastomer. Brandt, U.S. Pat. No. 4,010,054, "Thermoplastic Filament Winding Process", relates to a method of forming pipe by winding thermoplastic filaments into a rotating mandrel; and U.S. Pat. No. 4,357,962 to Shaw, et al, "Method and Apparatus for Producing Tubular Article", describes the formation of a composite pipe having a tubular liner formed of thermoplastic material with an outer layer of glass fiber fabric. An additional outer layer of fiberglass and resin is applied afterwards.
The present invention provides a process of producing a GRP pipe with a thermoplastic liner in a truly continuous process and without the use of an adhesion-promoting layer which is fully compatible with potable water. There are examples of disclosures of continuously formed thermoplastic lined thermoset resin pipe manufactured using a continuous mandrel; unlike the present invention, however, the methods disclosed in previous patents differ from the present invention in a number of ways. For example, patents such as U.S. Pat. No. 3,979,250 to Drostholm, "Apparatus For Making Continuous Lengths Of Resin Tubes", U.S. Pat. No. 4,011,354 and its continuation U.S. Pat. No. 4,081,302, use an intermediate bonding layer consisting of either materials pressed into the soft formed inner thermoplastic lining or grooves or other impressions made into the lining to provide good adherence to the thermoset resin shell. As a result, the pipes have not been useful on a prolonged basis when continuously exposed to water. There is a need for a pipe having an ultra-thin thermoplastic lining to secure an enhanced resistance to water with a higher safety margin against any possible contamination of potable water, at an affordable cost, making the pipe fully compatible for use with potable water applications. This need is met by the present invention.